Evidence Of Earth's Early Years May Still Lurk In The Mantle
(Inside Science) -- Some 120 million years ago, one of the largest volcanic eruptions in Earth's history sent lava flooding out of fissures in the seafloor, north of what is today the Solomon Islands in the Pacific Ocean. When the lava cooled it formed a vast underwater plain about the size of Alaska and nearly 20 miles thick known as the Ontong Java Plateau.
And while 120 million years old may seem ancient enough to make your bones ache, a new study published in the journal Science suggests this geologically young rock could provide a window to the Earth as it was nearly 4.5 billion years ago.
These rocks, along with others from Baffin Island off the coast of northern Canada, could give scientists insights into the Earth's earliest days, helping them better understand how the planet and perhaps even the moon formed. In addition to illuminating the young Earth, the research could potentially reshape our understanding of the internal workings of the planet including how the mantle mixes.
The finding is "very surprising," said co-author Hanika Rizo, a geochemist at the University of Quebec in Montreal.
Scientists have long concluded that rocky planets like Earth formed in the first 50 million years or so after the birth of the solar system about 4.5 billion years ago. As the planet formed through the addition of smaller bodies by impacts, enormous amounts of energy were released. Consequently, much of the young Earth melted, causing its heaviest metals like iron and nickel to sink and create the core, while the lighter, rocky material of the mantle floated above.
This separation event was a seminal moment in the early life of our planet. But how and when it occurred and how it affected the chemical cocktail of the mantle have remained largely mysteries. Understanding those early processes are crucial, in part, because they could yield important insights into how our planet came to develop and sustain life, said Rizo.
But until recently scientists had thought that any chemical clues of the early Earth's mantle would have been long gone. That's because, they assumed, that early cataclysmic events like the giant impact that formed the moon as well as processes like plate tectonics and convection in the mantle would have thoroughly mixed the mantle over time, erasing the elusive primordial geochemical fingerprint the scientists sought.
But then several years ago in the very oldest rocks found on Earth -- some of which date back nearly four billion years -- they uncovered a clue: traces of a geochemical signature that only could have formed during roughly the first 50 million years of Earth's history. Using that signature, the scientists could finally begin to access the planet's most distant past.
One part of this signature is the abundance of different forms of the element tungsten in a rock. Besides making up part of the filaments in incandescent light bulbs, tungsten has also proven particularly useful for peering into Earth's early days. That's because the element has a particular isotope -- versions of an element that have a different number of neutrons in each atom -- that only forms when an isotope of another element known as hafnium radioactively decays.
As it so happens, most of this particular hafnium isotope decayed to this particular tungsten isotope during the first 50 to 60 million years of the solar system's history and as part of the processes that led to the formation of the Earth's core.
That means that "any variations in the tungsten isotopic composition that we're seeing in terrestrial rocks now are likely recording processes that occurred very, very early in solar system history," said co-author Richard Walker, a geochemist at the University of Maryland in College Park.
In the new study, the scientists weren't expecting to find such variations in the tungsten signature of relatively young rocks from the Ontong Java Plateau and Baffin Island, both of which are flood basalts -- the result of an enormous volcanic eruption or series of eruptions that coat large areas with basaltic lava. These types of eruptions have been linked to mass extinction events in the past.
The mantle sources of these younger rocks, the scientists thought, would have been exposed multiple times to the processes of plate tectonics and mantle convection, further obscuring any geochemical signature.
But the early chemical mark remained.
"The whole of my career I've been looking at old rocks," said Rizo. "Now looking at these modern rocks and finding they have these geochemical signatures is extremely exciting."
Like a time capsule, the rocks could preserve clues about how and when the Earth's core formed and how the mantle was altered as a result.
The findings, said Walker, "also suggest that whatever processes happened during this first 50 million years of Earth's history, some evidence of it has remained lurking in the mantle until essentially the present. And it's still down there somewhere."
That means that "the Earth doesn't mix itself as well as we had thought it did," he said. Figuring out how that primordial mantle has seemingly survived for all this time is the next big step.
What's more, the result also suggests that such ancient mantle regions, which formed before the giant impact that led to the formation of the moon, would have survived the cataclysmic event. That's at odds with some models of how the moon was created, which predict that the impact would have melted the entire Earth and homogenized the chemistry of the mantle, said Rizo.
The finding is "very interesting if it's right," said Tim Elliott, a geochemist at the University of Bristol in England who wasn't involved in the study. The Earth is a dynamic planet, he said, and through processes like plate tectonics and mantle convection, it resurfaces three-quarters of itself every 200 million years. "That does a pretty good job of erasing vestiges of its early history. So the fact that you can potentially see back to the very beginning is quite nice."
But, for the moment, he's skeptical. That's, in part, because in previous research, Elliott and his colleagues performed similar analyses on rock samples from the Ontong Java Plateau and found no such tungsten anomalies.
Measuring more flood basalts and other basalts from materials related to mantle plumes will help scientists figure out if the new findings are right, he said.
If the new research stands up, "what we're potentially seeing here is material that was processed very early on in Earth's history," he said. "It did nothing for 4.5 billion years, and then made its way up 3,000 kilometers [the distance from roughly the base of mantle to the crust], melted, and eventually formed these rocks. And now in their composition, we're seeing that whole history . . . To me, that is incredibly exciting."